SiGe (Ge-dot) heterojunction phototransistors for efficient light detection at 1.3–1.55μm

doi:10.1016/S1386-9477(02)00634-3

Physica E: Low-dimensional Systems and Nanostructures

Volume 16, Issues 3–4, March 2003, Pages 528-532

https://doi.org/10.1016/S1386-9477(02)00634-3 Get rights and content

Abstract

The aim of this work is to develop a Si/SiGe HBT-type phototransistor with several Ge dot layers incorporated in the collector, in order to obtain improved light detectivity at 1.3– $1.55 μm$ . The MBE grown HBT detectors are of n–p–n type and based on a multilayer structure containing 10 Ge-dot layers (8 ML in each layer, separated by $60 nm$ Si spacer) in the base-collector junction. The transistors were processed for normal incidence or with waveguide geometry where the light is coupled through the edge of the sample. The measured breakdown voltage, BV_ceo, was about $6 V$ . Compared to a p–i–n reference photodiode with the same dot layer structure, photoconductivity measurements show that the responsivity is improved by a factor of 60 for normal incidence at $1.3 μm$ . When the light is coupled through the edge of the device, the detectivity is even further enhanced. The measured photo-responsivity is more than 100 and $5 mA/W$ at 1.3 and $1.55 μm$ , respectively.

Introduction

Most of the integrated circuits are until now based on Si because of its well-established processing technology and low cost. However, Si is an indirect band gap $(E_{g} =1.12 eV)$ material, resulting in poor optical properties such as emission and absorption of light. The implementations of self-assembled Ge quantum dots (QD:s) in Si have thus during recent years attracted a significant interest, since these systems are promising for manipulating the properties of Si for photonic applications (for a review see Ref. [1]). The Stranski–Krastanov growth mode is widely used to fabricate Ge QD:s in Si, where the growth mode is changed from a 2-D to a 3-D island formation without introduction of misfit dislocations [2], [3].

Due to the smaller band gap of these nano-sized particles, the carriers will be confined in the dots with discrete energy levels. The band alignment between the incorporated Ge dots and the Si host material is known to be of so-called type II, meaning that holes are confined in the Ge QD:s, while the lowest potential for the electrons in the conduction band is in the Si. The band-to-band transition energy is significantly lowered in this case, shifting the absorption edge to an energy of $∼0.8 eV$ . This makes the material useful for photonic applications at wavelengths in the range of 1.3– $1.55 μm$ , which is of great interest for fiber optical communications since the attenuation in an optical fiber has minima at these wavelengths.

Improved light detection in the near infrared range has been reported in Si based p–i–n diodes with Ge QD:s in the intrinsic region both at normal incidence [4], and with waveguide geometry [5]. In this communication we report a study on fabrication and characterization of a new type of photodetectors based on an n–p–n heterojunction bipolar transistor (HBT) with Ge islands embedded in the base-collector (bc) junction. A comparison is also made between the photo-responsivity of the HBT, and p–i–n photo-diodes with the same dot layer structure. Samples have been processed for both normal incidence and waveguide geometry with the incident light beam coupled through the device edge. A significant enhancement of the photo-responsivity has been obtained from the HBT detectors, which is a factor of 60 at $1.3 μm$ and 25 at $1.55 μm$ , respectively.

Section snippets

Experimental

The samples were grown in a Balzers UMS630 solid-source molecular beam epitaxy (MBE) system, with Si and Ge fluxes supplied by e-gun evaporators. A detailed description of the MBE system has been reported elsewhere [6]. After thermal cleaning of the sample at 825°C for $10 min$ , in situ reflection high-energy electron diffraction (RHEED) measurements showed good crystalline surface quality, evidenced by a sharp 2×1 surface reconstruction. The HBT structure was grown from the emitter towards the

I–V characteristics

Typical dark current I–V characteristics from a $0.5×0.5 mm^{2}$ p–i–n diode and a HBT photo detector are shown in Fig. 2. The reverse breakdown voltage for the p–i–n diode is $>15 V$ , with a low dark current density in the order of $0.9 mA cm^{−2}$ at $3 V$ . The breakdown voltage between the emitter and collector with an open base circuit (BV_ceo) for the HBT is smaller, $∼6 V$ , which is due to the current gain. The dark current density in this case is about $0.6 mA cm^{−2}$ at $3 V$ .

Responsivity at 1.3 μm

Fig. 3 shows the photo-responsivity at

Summary

In conclusion, an obvious enhancement of the light responsivity has been observed using a heterojunction bipolar transistor containing Ge quantum dots for light detection in the near infrared range. The devices show low dark current with BV_ceo of about $6 V$ . Compared to a reference p–i–n diode with a similar dot layer structure, the responsivity at normal incidence is a factor of 60 higher at $1.3 μm$ and 25 times higher at $1.55 μm$ for the phototransistor. The improvement is associated with the

Acknowledgements

The work has been supported by the Swedish Foundation for Strategic Research (SSF).

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SiGe (Ge-dot) heterojunction phototransistors for efficient light detection at 1.3–1.55μm

Abstract

Introduction

Section snippets

Experimental

I–V characteristics

Responsivity at 1.3 μm

Summary

Acknowledgements

J. Crystal Growth

Rep. Progr. Phys.

Phys. Rev. Lett.

SiGe (Ge-dot) heterojunction phototransistors for efficient light detection at 1.3– $1.55 μm$